1. Field of the Invention
The present invention relates to a fixing apparatus of electrophotographic recording in which the power required for fixing is reduced.
2. Description of the Related Art
An electrophotographic recording apparatus has features such that it produces less noise as it is a non-impact recording apparatus, it can record characters clearly, its recording speed is high, and its running cost is relatively low. Therefore, it is recently used as an output terminal of OA equipment. The market of the electrophotographic recording apparatus is also quickly widening.
FIG. 1 is a schematic diagram of a recording apparatus of a laser printer as an example of the electrophotographic recording apparatus. The outline of the electrophotographic recording apparatus will be described. A conventional electrophotographic recording apparatus uses a photosensitive drum 100, as shown in FIG. 1. The entire surface of the photosensitive drum 100 is first uniformly charged to, e.g., about -700 V with negative charges by a charging apparatus 101 comprising a corona charger. Then, the photosensitive drum 100 is irradiated with a laser beam 102 in accordance with an image signal. The resistance of a portion of a photosensitive material which is irradiated with the beam is decreased. Therefore, an electrostatic latent image in which the negative charges are erased is formed at the portion irradiated with the laser beam 102. Usually, one semiconductor laser is used as the laser, and the beam modulated in accordance with the image performs scanning by a rotating polygonal mirror.
The electrostatic latent image formed in this manner is then developed by a developing apparatus 103. That is, when a developing bias of about -500 V is applied to the developing roller, a toner as fine color particles charged with, e.g., negative charges by reversal development attaches to the portion of the electrostatic latent image on the photosensitive drum 100 in which the negative charges are erased, thereby visualizing the latent image. Then, recording paper 105 which is picked up by paper feed rollers 104 from a paper cassette (not shown) is conveyed in synchronism with an image signal, and contacts the photosensitive drum 100. Transfer of the visualized toner image to the recording paper 105 is performed. In a transfer charger 106, positive charges are applied to, e.g., the lower surface of the recording paper 105. Thus, the negatively charged toner image on the photosensitive drum 100 is attracted to the recording paper 105 and transferred to it. The image-transferred recording paper 105 is then separated from the photosensitive drum 100 by a separation charger 107.
Finally, the toner is heated and pressed by a fixing apparatus 111 having a heat roller 110 and fixed on the recording paper 105, thereby completing recording. Note that part of the toner which is not transferred to the recording paper 105 remains on the photosensitive drum 100. The remaining toner on the drum 100 is cleaned by being scraped by a cleaner having a cleaning blade 108. Thereafter, the entire surface of the drum 100 is exposed to light by an erasure lamp 109 comprising an LED or the like, thereby erasing the charges on the photosensitive drum 100.
In this manner, in the electrophotographic recording apparatus, the steps of charging, formation of a latent image, development, transfer, and fixing are performed to form an image. The drum is cleaned in the cleaning step and is repeatedly used. Processes may differ more or less depending on the types of apparatuses but basically include the above steps.
The laser printer has been briefly described as a typical example of the electrophotographic recording apparatus. As the electrophotographic recording apparatus, not only a laser printer but also a recording head which uses other light-emitting elements for writing an electrostatic latent image have been developed and marketed. In the laser printer, a beam generated by one laser is radiated onto a polygonal mirror which rotates mechanically at a high speed or a hologram to scan the dots. A solid state scanning method which uses an array light source in view of reduction in system size and cost also currently has begun to attract attention. For example, an electrophotographic recording apparatus with a head in which light-emitting elements, e.g., LEDs, liquid crystal shutters, EL elements, plasma light-emitting elements, and fluovescent dot array, or light-shutter elements are arranged in an array has been put into practical use. Either electrophotographic recording apparatus described above is generally called an optical printer and utilized as an output apparatus, e.g., a printer or a digital copying machine. Another example of the electrophotographic recording apparatus is an analog copying machine in which the original is irradiated with light from, e.g., a conventionally used fluorescent lamp, and the light reflected by the original is guided to the photosensitive member to form an electrostatic latent image, thereby copying the original. Furthermore, another recording method called ion flow recording or ion deposition recording is also available. According to this method, a dielectric material is used in place of a photosensitive member. Ions are blown from pores formed in an array manner. An electrostatic latent image is formed on the dielectric material by the ions.
As has been described above, since the electrophotographic recording apparatuses have excellent features, they are often utilized recently as output terminal apparatuses of OA equipment. Various methods for them are developed and put into practical use, with their market rapidly expanding.
In these electrophotographic recording apparatuses, recording is performed in the common steps of charging, formation of a latent image, development, transfer, and fixing, as described above. One of the characteristic features of the electrophotographic recording apparatus is that a very low energy is required for forming an electrostatic latent image. For example, to form a latent image of one dot, a light energy of as small as about 10.sup.-6 to 10.sup.-5 J/cm.sup.2 is applied to the photosensitive member. In contrast to this, to form one dot on a recording material by, e.g., a thermal transfer recording apparatus, a large recording energy of about 2 to 6 J/cm.sup.2 is required. If only these facts are considered, an electrophotographic recording apparatus seems to have a very high efficiency and its power consumption seems to be very low compared to those of a thermal transfer recording apparatus.
In an actual electrophotographic recording apparatus, however, the power consumption is normally about 1.5 Kw in an apparatus which can record 8 to 12 sheets per minute, and is about 500 to 600 W at minimum in a low-speed apparatus which can record 4 sheets per minute. These values are of the same level or higher when compared with a thermal transfer recording apparatus. In the electrophotographic recording apparatus, some of the recording processes from charging to transfer of a toner image on plain paper are certainly realized with a very low energy. However, a high energy is consumed in the final step of fixing the toner to the recording material. Therefore, the power consumption as a whole in the electrophotographic recording apparatus is increased. For example, the fixing energy is as large as about several tens of J/cm.sup.2, which is about ten times the recording energy of the thermal transfer recording apparatus.
Most electrophotographic recording apparatuses use a heat roll type fixing apparatus which performs fixing by heat and pressure. The fixing apparatus which uses a heat roll is safe as it is free from a danger such as ignition. Since the heat capacity is large, stable image quality can always be obtained. The fixing intensity is sufficiently higher than that of pressure fixing and the like.
However, since the heat roll has a large heat capacity, it takes time to increase the temperature of the heat rolls to a value required for fixing. Therefore, the apparatus cannot be used immediately after turning on the switch, and a warm-up time of about several minutes is usually necessary. Since the heat roll has a large heat capacity, a heater having high power consumption is needed, and thus, e.g., an infrared lamp of about 500 to 1,000 W is normally incorporated in the roller. In fine, the conventional electrophotographic recording apparatus uses a heat roll having a large heat capacity as the fixing apparatus. Therefore, although it has several advantages, it also has disadvantages in that high power consumption is needed and the warm-up time is long. When size reduction of the electrophotographic printing apparatus is considered, it is not preferable to use, as the fixing apparatus, a heat roll which has high power consumption and generates much heat.